Datasheet

LM27313
www.ti.com
SNVS487D DECEMBER 2006REVISED APRIL 2013
APPLICATION INFORMATION
SELECTING THE EXTERNAL CAPACITORS
The LM27313 requires ceramic capacitors at the input and output to accommodate the peak switching currents
the part needs to operate. Electrolytic capacitors have resonant frequencies which are below the switching
frequency of the device, and therefore can not provide the currents needed to operate. Electrolytics may be used
in parallel with the ceramics for bulk charge storage which will improve transient response.
When selecting a ceramic capacitor, only X5R and X7R dielectric types should be used. Other types such as
Z5U and Y5F have such severe loss of capacitance due to effects of temperature variation and applied voltage,
they may provide as little as 20% of rated capacitance in many typical applications. Always consult capacitor
manufacturer’s data curves before selecting a capacitor. High-quality ceramic capacitors can be obtained from
Taiyo-Yuden, AVX, and Murata.
SELECTING THE OUTPUT CAPACITOR
A single ceramic capacitor of value 4.7 µF to 10 µF will provide sufficient output capacitance for most
applications. For output voltages below 10V, a 10 µF capacitance is required. If larger amounts of capacitance
are desired for improved line support and transient response, tantalum capacitors can be used in parallel with the
ceramics. Aluminum electrolytics with ultra low ESR such as Sanyo Oscon can be used, but are usually
prohibitively expensive. Typical AI electrolytic capacitors are not suitable for switching frequencies above 500
kHz due to significant ringing and temperature rise due to self-heating from ripple current. An output capacitor
with excessive ESR can also reduce phase margin and cause instability.
SELECTING THE INPUT CAPACITOR
An input capacitor is required to serve as an energy reservoir for the current which must flow into the inductor
each time the switch turns ON. This capacitor must have extremely low ESR and ESL, so ceramic must be used.
We recommend a nominal value of 2.2 µF, but larger values can be used. Since this capacitor reduces the
amount of voltage ripple seen at the input pin, it also reduces the amount of EMI passed back along that line to
other circuitry.
FEED-FORWARD COMPENSATION
Although internally compensated, the feed-forward capacitor Cf is required for stability (see Typical Application
Circuits). Adding this capacitor puts a zero in the loop response of the converter. Without it, the regulator loop
can oscillate. The recommended frequency for the zero fz should be approximately 8 kHz. Cf can be calculated
using the formula:
Cf = 1 / (2 x π x R1 x fz) (1)
SELECTING DIODES
The external diode used in the typical application should be a Schottky diode. If the switch voltage is less than
15V, a 20V diode such as the MBR0520 is recommended. If the switch voltage is between 15V and 25V, a 30V
diode such as the MBR0530 is recommended. If the switch voltage exceeds 25V, a 40V diode such as the
MBR0540 should be used.
The MBR05xx series of diodes are designed to handle a maximum average current of 500mA. For applications
with load currents to 800mA, a Microsemi UPS5817 can be used.
LAYOUT HINTS
High frequency switching regulators require very careful layout of components in order to get stable operation
and low noise. All components must be as close as possible to the LM27313 device. It is recommended that a 4-
layer PCB be used so that internal ground planes are available.
As an example, a recommended layout of components is shown:
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